The present invention relates to telecommunications devices and, in particular, to an improved automatic power control loop for a wireless telecommunications transmitter.
The Federal Communications Commission (FCC) regulates the use of the radio frequency (RF) spectrum in the United States. Users of allocated bandwidth of the RF spectrum must take measures to ensure that radiated emissions inside and outside the allocated bandwidth are maintained within acceptable levels to avoid interfering with other users' operating in the same or other bandwidths. For example, users of cellular telephone systems must ensure that they are compliant with the level of radiated emissions allowable inside or outside the channels they have been assigned.
Cellular telephones such as, for example, CDMA (code division multiple access) or TDMA (time division multiple access) cellular telephones, include power amplifiers in the transmitter in which the power amplifier can be driven beyond a point where acceptable out of channel emissions are maintained. This is primarily due to the increased distortion output levels of the power amplifier at high powers.
Thus, regulating the transmitted signal power can reduce the amount of interference and spectral regrowth to a desired level. Certain wireless telecommunications transmitters, such as those used for cellular telephony, employ a transmit power control loop to regulate the transmitted signal power. In the mobile radio standard IS-95, for example, poorly regulated transmit power at the wireless terminal can lead to near-far effect at the base station demodulator, thus degrading the performance of the system. Similarly, in IS-136, drift in the transmit power loop can cause additional interference in the uplink channel, as well as spectral regrowth.
Regulating the transmitted signal power requires measurement or estimation of the actual transmitted signal power. However, this can be difficult, because modulation schemes such as DQPSK introduce signal power variations on the order of several decibels. On the digital side, this is due largely to the use of a square root raised cosine filter for pulse shaping as is required by the IS-136 standard, for example. FIG. 1 illustrates an exemplary instantaneous transmit power plot. As can be seen, the instantaneous transmit power fluctuates between −4 and −22 dB for eight times oversampling. FIG. 2 illustrates the corresponding signal constellation. Again, there is considerable fluctuation about the points of interest. The power is constant only is sampled at the maximum symbol peak.
Furthermore, components in the transmit chain, and particularly the power amplifier, tend to have a relatively wide variation in gain with respect to unit, temperature and frequency. Maintaining a given output power over all units, temperature, and frequency would typically require a multidimensional calibration table. This requires additional memory and software and, hence, adds to the cost of the telephone.